Dielectric wave guide system



June 17,1947, M, D, FISKE, 2,422,189

DIELECTRIC WAVE GUIDE SYSTEM Filed Jan. 22, 1944 Inventor: Miiah D.Fiske,

' His Attorney.

Patented June 17, 1947 DIELECTRIC WAVE GUIDE SYSTEM Milan D. Fiske,Schenectady, N. Y.,- assignor to General Electric Company, a corporationof New York Application January 22, 1944, Serial No. 519,267

Claims.

My invention relates to dielectric wave guides of the hollow pipe typefor transmitting electromagnetic waves of ultra high frequency and, moreparticularly, to structures associated therewith for sectionalizing sucha guide to permit operation of adjacent sections under differentpressure conditions.

It is known that electromagnetic waves may be propagated through theinterior of la hollow pipe type guide constructed of a conductivematerial-and containing a dielectric medium, such as air or gas, throughwhich the energy of the wave is transmitted. The frequency of theexciting electromagnetic waves, of course, must be greater than thecritical minimum frequency established by the dimensions, principallythe transverse dimensions, of the guide.

There maybe placed within the interior of a dielectric guide a, metallicwall having an aperture which is tuned to the frequency of the excitingwaves to effect a concentration of the potential due to the waves withinor across a substantially small dimension of the aperture. If thintensity or magnitude of the exciting electromagnetic waves isincreased, the magnitude of the potential diiference appearing betweenopposing surfaces of the aperture is also increased and, when themagnitude ofthis potential difference increases to a predetermined valueestablished by the configuration of the aperture and dielectric strengthof the medium surrounding the aperture, an electric discharge isestablished across the aperture thereby materially changing thepropagation characteristics of the guide. A structure of this nature maybe employed, for example, in connection with an ultra high frequencysystem which is subjected to electromagnetic waves of considerable rangeof intensity and which utilizes the characteristics of the aperture toeffect transmission or attenuation of the Waves within the guide.

It is an object of my invention to provide a new and improved dielectricwave guide of the hollow pipe type in which a transverse metallic wall,having a resonant aperture, is provided with a window to seal saidaperture to permit operation of the regions on opposite sides of saidwall at diiferent pressures.

It is a further object of my invention to provide a new and improvedmeans for sectionalizing a wave guide.

It is still another object of my invention to provide a new and.improved window for a Wave guide which is substantially reflectionlessover a 2 wide frequency band of the electromagnetic waves beingpropagated along said guide. I

It is a still further object of my invention to provide a new andimproved resonant aperture enclosed within or associated with a waveguide of the dielectric type and which is provided with a window havingsubstantially zero phase extension along the guide.

It is a still further object of my invention to provide a new andimproved resonant structure associated with the dielectric wave guidewhich has small frequency sensitivity.

It is still another object of my invention to provide a wave guide ofthe hollow pipe type with a glass window having a reflectioncharacteristic similar to that of a single tuned parallel circuit.

It is another object of my invention to provide a new and improvedwindow structure associated with a dielectric wave guide which is highlyresponsive to the magnitude of the exciting electromagnetic waves.

It is a still further object of my invention to provide a, new andimproved window for a wave guide having means to prevent sputtering ofthe glass surface by electrodeless discharge.

One of the features of my invention is the provision in a metallic wallfor sectionalizing a wave guide of a resonant aperture, such as aresonant slot, tuned substantially to the frequency of the excitingelectromagnetic waves and which is designed toeffect a breakdown of thesurrounding medium in the event the intensity of the electromagneticwaves attains or exceeds a predetermined value. The presence of theelectric discharge across the slot varies the effective dielectricconstant of the dielectric medium through which the electromagneticwaves are propagated thereby changing the Wave guide from a propagatorto an attenuator of the electromagnetic waves.

incorporated in a hollow pipe dielectric wave guide; Fig. 2 is across-sectional view of the A seal is provided across the resonantaperture and may comprise a vitreous dielectric window structure for theresonant aperture or slot in a wave guide; and Fig. 3 is a view of amodification of the wave guide window of my invention.

It is known that ultra high frequency electromagnetic WaVes may bepropagated dielectrically through hollow pipe type guides where thefrequency of the exciting electromagnetic waves is greater than acritical minimum frequency which, in turn, is determined principally bthe transverse dimensions of the guide. Such guides pref,- erably areconstructed of aconductive material, such as copper or brass. of highfrequency electromagnetic waves, may be transmitted dielectricallythrough hollow pipe type guides. Of these waves, the transverse magneticor TM type waves have both a longitudinal and a transverse component ofelectric field but only a, transverse component of magnetic fieldwhereas the transverse electric or TE type waves have both alongitudinal and a transverse component of magnetic field but only atransverse component of electric field. Although my invention isapplicable to systems for transmitting a great variety of waves, indescribing my invention hereinafter reference will be made particularlyto the TEM type of wave.

Referring now to Fig; 1 of the accompanying drawings, my invention isthere illustrated as applied to a system including a hollow pipe typewave guide of the dielectric type. The dielectric wave guide may beofrectangular cross section formed by conductive material, such ascopper or brass, having a height a and a base dimension D.Electromagnetic waves are established within the guide by suitableexciting electrodes which may have variousconfiguratioris depending uponthe nature 'of the excitation required or desired. One form which theinput electrode or exciting means may assume is that of a concentricline including tubular outer conductor 1 and an inner conductor 2, theformer of which is conductively connected to the bottom of the waveguideand the latter of which may be conductively connected to the top. Thereis positioned within the wave uide a metal wall 3 which may beconstructed of a suitable alloy, such as an iron-nickel-cobalt alloy.The wall 3 is provided with an aperture 4 having an appreciabledimension transverse to the direction of propagation of electromagneticwaves within the guide. Where the wave guide is excited byelectromagnetic waves of the TEm type, for example, the slot affords aprincipal dimension transverse to the electric component of theelectromagnetic field and effects a concentration of the potential dueto the wave across the upper and lower edges of the slot. Thisaccentuation or concentration of the potential due to the wave iseffected by virtue of the fact that the slot is tuned to the frequencyof the exciting waves, i. e., causes little reflection ofelectromagnetic waves of this frequency. If desired, the resonantaperture may be of the configuration .illustrated wherein the slot 4 isrectangular in shape. A resonant slot of this type is describedingreater detail and claimed in my copendin application. Serial No.458,422, filed September 15, 1942, and assigned to the assignee of thepresent invention.

In ultra high frequency transmission systems which employ dielectricwave guides of th hollowpipe type, it is desirable frequently to operateadjacent sections of a, wave guide at different pressures. atmosphericpressure while the adjacent section is either evacuated or filledwith asuitable gas at 4 a pressure other than atmospheric. To this end, in thesystem'illustrated in Fig. 1, the complete opening or aperture 4 issealed by means of a dielectric window 5 constructed of a suitablevitreous material, such as a boron-silicate glass, having substantiallythe same coefiicient of linear expansion as the alloy material of whichthe transverse wall3 is constituted.

The wall 3 and the associated dielectric window constitute one end of asealed chambe or section of the dielectric wave guide, the longitudinalend of such section being provided by the end wall Moreover, many typesThus, one section may be operated at beyond the concentric electrodemeans I and 2.

It is to be understood that the concentric line may glass ishermetically sealed in place, the window may be ground on both sides toobtain precise dimensions so that the surfaces of the glass 5 arecoplanar with the surfaces of the wall 3. So constructed, the window 5lies nearly in a single electrical plane, that is, it has very littlephase extension along the guide. Hence, the effect of the window 5 andthe resonant slot 4 can be compared to a single resonant circuit in atransmission line so far as their reflection properties are concerned.The thin element constituted by the window 5 and the resonant slot 4behaves as a parallel resonant circuit shunted across the line, thiscircuit having negligible resistance because of the high conductivity ofthe metal wall 3 and low dielectric loss in the glass 5. It has beenfound that such an element can be made almost completely refiectionlessover a wide band of frequencies. Theeffect, therefore, of such a thinelement is considerably different from an element having a substantialphase extension along the wave guide, for such an element would berepresented by two or more circuits of finite phase separation and, ingeneral,'reflection from sucha combination would not vanish at anyfrequency.

For a thin composite element comprising the metal wall 3 and a glasswindow 5 sealed across the gap 4, it has been found that the reflectionversus frequency curve of an incident electromagnetic wave is almostidentical with the ordinary tent the exact resonance frequency, et gen-uerally speaking, as the effective dimensions of the aperture transverseto the electric component of the field is increased, the frequency towhich the aperture is resonant is decreased. At the same time, the phaseextension of the composite window structure increases with the height ofthe resonant slot 4.

.The voltage Or Potential difference appearing between the upper andlower edges of the aperture 4 is eifected by the resonantcharacteristics of the aperture and the magnitude of this voltagedifference increases as the magnitude of the exciting waves increases.Upon reaching a predetermined value, the dielectric material or mediumadjacent to the window 5 across the aperture breaks down, that is, thevoltage difference is sufficient to cause ionization of the medium andan electric discharge takes place across the aperture 4. Upon occurrenceof such an electric discharge the wave transmitting characteristics ofthe guide are radically changed causing rapid attenuation of theelectromagnetic waves within the vicinity of the wall 3. The presence ofthe charged particles within the vicinity of the resonant apertureduring the electric discharge effectively changes the dielectricconstant of the medium through which the waves are being propagatedthereby substantially increasing the critical minimum frequency for theparticular guide employed so that the waves are not propagated anappreciable distance beyond the wall 3. I

The voltage or potential difference appearing between the upper andlower edges of slot 4 effects a concentration of electric field acrossthe slot and across the window 5. Where the window is used to separatetwo wave guide sections having different pressure conditions, it hasbeen found that as the potential diiference across the slot 4 increases,an electrodeless discharge, i. e., an electric discharge which takesplace by virtue of the electric field strength being suificient toproduce ionization without the presence of any electrodes, occurs acrossthe window 5 on its lower pressure side. In Fig. 3 there is shown amodification of my invention wherein means is provided to reduce oreliminate sputtering onto the glass window 5 produced by thiselectrodeless discharge. This sputtering arises whenever the discharge,the area of which varies with the strength of the incidentelectromagnetic energy, becomes great enough that it extends over theentire window 5 and touches any portion of the metallic wall 3. layer ofglass 1 connected to the glass window 5 and extending over the portionof the metal wall 3 adjacent to the aperture 4. The layer 1 has aminimum thickness and extends a substantial distance over the outersurfaces of wall 3 toward the walls of the wave guide.

In constructing a wave guide system embodying my invention, the sectionIn may be provided at its end with a shoulder ll, against which thetransverse wall 3 abuts. guide, at its end connecting with section l0may be provided with an integral sleeve I 3 which slips over the outersurface of section In so that wall 3 is held firmly between adjacentedges of sections l0 and I2.

From the foregoing description, it is thus seen that my inventionprovides a new and improved window for sealing adjacent sections of awave guide having differential pressures therebetween, the window havinga small phase extension and causing a minimum reflection of theelectromagnetic wave being propagated along the guide. At the same time,the composite structure has a low frequency sensitivity, that is,provides efficient transmission ove a substantial band of frequencies ofthe electromagnetic wave. The window, furthermore, may be employed as abreakdawn element if the medium on one side thereof is readilyionizable. When electromagnetic waves of sufficient intensity imping onthe window the This means comprises a glaze or thin The section I2 ofthe F medium, which may be a low-pressure gas, breaks down and causes avery considerable attenuation of the waves passing through the window.

While I have shown and described my invention as applied to oneparticular embodiment thereof, it will be obvious to those skilled inthe art that changes and modifications may be made withoutdeparting frommy invention, and I therefore intend in the appended claims to cover allsuch changes and modifications as fall within the true spirit and scopeof my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A combination, a dielectric wave guide of the hollow-pipe type andcomprising a pair of adjacent sections having differential pressurestherein, exciting means for establishing electromagnetic waves withinsaid guide, and means for transmitting said waves between saidsectionssubstantially without reflection, said means comprising ametallic wall associated with said guide and lying between said sectionswithin a plane substantially transverse to the direction of wavepropagation through said guide, said wall being provided with anaperture having an appreciable dimension perpendicular to the electriccomponent of field incident to the propagation of waves through saidguide, and vitreous means sealed across said aperture and substantiallyco planar with said wall, said aperture and said vitreous means beingtuned to the frequency of said exciting means and having substantiallyzero phase extension along said guide.

2. In a dielectric wave guide of the hollowpipe type and including asubstantially closed section, exciting means for establishingelectromagnetic waves Within said section, and means for facilitatingtransmission of said waves from said section substantially withoutreflection, said means comprising a metallic wall defining a boundary ofsaid section and lying in a plane substantially transverse to thedirection of wave propagation through said guide, said wall beingprovided with an aperture having an appreciable dimension perpendicularto the electric component of the field incident to the propagation ofwaves through said guide and tuned to the frequency of said excitingmeans, and a glass window hermetically sealed across said aperture andsubstantially coplanar with said wall, said window and said aperturehaving a minimum phase extension along said wave guide.

3. In combination, a dielectric wave guide of the hollow-pipe type,excitin means for establishing electromagnetic waves within said guide,a metallic wall associated with said guide and lying in a planesubstantially transverse to the direction of wave propagationtherethrough provided with an aperture having an appreciable dimensionperpendicular to the electric component of the field incident to thepropagation of waves through said guide, and a vitreous member sealedacross said aperture, said vitreous member having a portion extendingover the surface of said wall adjacent said aperture to pre- Vent injuryto said member produced by an electric discharge established across saidaperture in the presence of electromagnetic waves of substantialintensity.

4. In combination, a dielectric wave guide of the hollow-pipe type,exciting means for establishing electromagnetic waves within said guide,a metallic wall associated with said guide and lying in a planesubstantially transverse to the direction of Wave propagation throughsaid guide mans by $1 6 am; aimrwradu t6 elegs of S ibSt'afitiaI itegifslti}, sa d I mgle; thihlay r ofrilf sulaitlin 'g t'e'rilaovgfiiggsa'i djwall adjacent samfajpe tui e.

inatidfija di lect ic wa e i dewf theiiblldw p pe; 1331p? inq'comppismgp51; oi qqljace nfi 's ect'ibhs' having difie feritji l prgsures thel'ebtwen, exitirig mea 'r is fo i' Establishing elgdigiqr'ngpetic waveswithin said guidg, and f0: tfgtmxpitbing said waves bgtwegp said seict'i ris 'subs taht ially Without reflectipn, said me "risfcombri'si'xig metallic wall associated with ,sg d; guide andlyingbetwgen saidsgc'tion s within afplan e s i 1btaliitia11y transverse to thedirecjqign 6f. wav b pgtion through aid guideysa id wan q roviq q w th'an a rtu e 'avin fan a r ia dim nsi qnz IK QQiQ W 't the e e REFEK NQETE? V lThe following references are of record in th e file of thispatent:

STATES PAT NTS umb? Name t 7 2,129,713 Southworth Sept; 13; 19318:2,200,023 Dallenbach :May'7., 1940;

